1. Field of Invention
This invention relates to the output buffers driving a heavy output load with fast switch to minimize output delay but having the minimum power consumption, power surge and ground bouncing.
2. Description of Prior Art
The buffer is to isolate the functional circuit from the output load. In the CMOS digital circuit, the buffer is the inverter. The power surge and ground bounce are serious problems. In the switch of output buffer, there is a period both PMOS and NMOS being on. The power VDD is short to the ground GND. A lot of current flows from power to ground. It causes the ground bounce. To reduces the ground bounce and the power comsumption, the nonoverlapping switching mechanism is introduced. However, the nonoverlapping switch increases the output delay time. Ideally, we want to have both the nonoverlapping switch and the fast switch to minimize the output delay. So far, none of the references has these combinatory superior features. I make the innovative design to overcome the problem. My buffer is nearly the ideal buffer. It has the intelligence to smooth the current density to reduce the ground bouncing. It doesn't have the short circuit between the power and ground. The power consumption is minimized. The peak current is also minimized, too. In the buffer design, I adopt the feedback system theory and the analog circuit design techniques to design. My buffer is simple that it still keeps the high speed characteristics. None of the references has such a novel design.
The U.S. Pat. No. 4,777,389 issued to Wu et. al. on Oct. 11, 1988 which is incorporated herein by reference, used the logic circuit to generate the delay and control signal. The logic gates wastes the layout area and consumes a lot of power. The circuit speed is slowed down. The control is not continuous as the analog signal. The effect of reducing ground bouncing is doubtful. The U.S. Pat. No. 4,820,942 issued to Chan on Apr. 11, 1989 which is incorporated herein by reference, used two buffers, AC buffer and DC buffer. His buffer still uses the logic gates to introduce the delay. He uses 17 gates forty-six transistors in his buffer. Instead of reducing the ground bounce, the fourty-six transistors will consume a lot of power and induce a larger ground bounce. The layout area increases and the circuit speed is slowed down.
The U.S. Pat. No. 4,952,818 issued to Erdelyi on Aug. 28, 1990 which is incorporated herein by reference, used the output feedback to keep the PMOS or NMOS to be partially open and the switch speed is slowed down a lot. HIs circuit has two inverters to drive the PMOS and NMOS separately. On the contrary, in Erdelyi's invention, there is no nonoverlapping mechanism at all. Comapring with my invention, my invention has the nonoverlapping mechanism and output feedback fast switch; Erdelyi's invention doesn't have the nonoverlapping mechanism and the output feedback slows down the switch. My invention is completely different from Erdelyi's invention.
The U.S. Pat. No. 4,649,294 issued to McLaughlin on Mar. 10, 1987 which is incorporated herein by reference, used the output feedback to feed through the input signal to the pull down bipolar. His design completely violates the principle nonoverlapping switch and output feedback switch. My invention uses PMOS to be the output feedback device on the pull-down side; McLaughlin's invention used NMOS to be the output feed-back device on the pull-down side. It induces completely different result. As the output switch from high to low, in McLaughlin, the output feedback NMOS device turns on the pull-down bipolar first. On the contrary, the turning off signal for the pull-up bipolar go throgh one inverter and has much longer delay time. There is a much longer overlapping period to have the power short to the ground. The ground bounce is much larger. Furthermore, as the output switch from high to low, the feedthrough NMOS is disconnected. The switching speed decreases and the output delay increases. Comapring with my invention, my invention has the nonoverlapping mechanism and output feedback fast switch; McLaughlin's invention doesn't have the nonoverlapping mechanism. Even worse, in his invention, the switch overlapping time is much longer. Furthermore, in his invention, the output feedback slows down the switch. So, my invention is completely different from McLaughlin's invention.